Kerr cell shutter assembly



amqmaoT ff fr SEARCH ROOM Oct. 30, 1962 3,060,807

A. E. HUSTON KERR cram. SHUTTER ASSEMBLY Filed Sept. 22, 1959 INVENTORALEXANDER EDIIARD HUSTON ATTORNEYS United States Patent 3,060,807 KERRCELL SHUTTER ASSEMBLY Alexander Edward Huston, Reading, England,assignor to United Kingdom Atomic Energy Authority, London, EnglandFiled Sept. 22, 1959, Ser. No. 841,554 Claims priority, applicationGreat Britain Sept. 24, 1958 2 Claims. (Cl. 88-61) This inventionrelates to high speed cameras. High speed cameras, particularly therotating mirror-streak and cine cameras, have become establishedequipment for the photography of rapid events. They may be designed totake only single photographs or a series of photographs, and may have astationary or moving film. The exposure time must be kept very short,and electro-optical shutters have been proposed for this purpose.

The high speed cameras to which this invention relates haveelectro-optical shutters of the Kerr cell type in which a transparentcell is filled with a fluid (generally nitro-benzene). The fluid has thepower of splitting plane polarised light into two components when a highvoltage is applied across two electrodes immersed in it. When the cellis placed between two crossed poralisers, light will not pass throughthe system until the voltage is applied. The light-transmittingqualities of a nitrobenzene cell are not good, being only about 0.04 to0.15, and other liquids, e.g. phenyl cyanide, have been proposed in anattempt to improve the performance.

The voltage which is to be applied to the cell is about 30 to 50 kv. persq. cm. and this has, of course, to be applied only for the time neededfor the exposure. A single pulse can fairly easily be applied to a cellto give an exposure down to 0.1 micro-second, but the application of atrain of short pulses is more difficult and the difliculty increasessharply as the number of pulses per second increases.

One reason for the increase in difliculty is the large increase inenergy required by the electrical circuits at present used to produce asequence of pulses in a Kerr cell.

Up to now the greatest framing rate achieved in practice is about 2X 10per second.

This invention produces a high speed camera having an electro-opticalshutter which can operate at a framing rate above 2x 10 per second andhas only a relatively low electrical energy consumption.

The invention consists in a high speed camera having an electro-opticalshutter of the Kerr cell type, wherein the electrodes of the Kerr cellare connected across an inductance to form a parallel resonant circuit,means being provided to energise the circuit at substantially itsfundamental resonant frequency.

The means for energising the circuit may comprise a valve having thecircuit connected in its anode circuit.

In a preferred form of the invention the parallel resonant circuit isadapted to have a further resonant frequency of at least one harmonic ofthe fundamental.

The invention will be better understood with reference to theaccompanying drawings wherein:

FIG. 1 is a circuit diagram of a parallel resonant circuit wherein thepotential applied to the Kerr cell varies in unidirectional manner.

FIG. 2 illustrates the relationship between the mode of oscillation ofthe circuit in FIG. 1 and the operation of the cell.

FIGS. 3 to 5 illustrate the relationship between three modes ofoscillation and the shutter characteristics of a cell.

In FIGURE 1 a Kerr cell 1 (whose capacity is in the region 60 pf.) isplaced in series with an auxiliary capacitor 2 (of about 500 pf.). Thecell 1 and the capacitor 2 are connected in series across an inductance3. A diode 4 is connected across the cell 1 with its anode 5 tapping theconnection between cell 1 and capacitor 2. A radio frequency amplifiervalve 6 has its anode 7 connected to the inductance.3 and its cathode 8connected to earth. A 3 kv. input line 9 is connected between theinductance 3 and the cell 1. A capacitor 10 is connected in parallelwith cell 1 to capacitor 2 and taps the inductance 3 at an intermediatepoint in the inductance.

In FIGURE 2 the solid line 11 illustrates the light transmittingcharacteristic of the cell under the influence of an applied voltageillustrated by dotted line 12. The voltage illustrated by 12 can berepresented by the equation:

v=O.48 V. (sin wt.0.44 cos 2 wt.+0.65)

wherein v is the varying voltage applied to cell 1, V is the voltagerequired to open the cell 1 completely, w is the angular frequency ofthe oscillations and t is time.

In FIG. 3 the solid line 13 shows the light transmitting characteristicsof a cell under the influence of an applied voltage represented bydotted line 14. The applied voltage follows a simple sinecurve havingthe equation:

v=Vsin wt.

The circuit of FIG. 1 would be modified to produce these sinusoidaloscillations by removal of the capacitor 2, the diode 4 and thecapacitor 10.

In FIG. 4 the varying voltage 15 is governed by the equation:

21: (sin wt.sin 3 wt.)

The light transmission characteristics of the cell are shown by line 16.The circuit illustrated in FIG. 1 would be modified to oscillate in thismanner by removal of the capacitor 2 and the diode 4. I

In FIG. 5, the varying voltage represented by line 17 is governed by theequation:

The light transmitting characteristics of the cell under the influenceof the applied voltage 17 are shown 'by line 18.

The circuit shown in FIG. 1 would be modified to oscil-- late in thismanner by removal of capacitor 10. The circuit of FIG. 1 operates asfollows:

Valve 6 is operated as a class C R.F. power amplifier.

It is fed on its control grid with a signal of the funda mentalfrequency and as with any class C amplifier the anode current is rich inharmonic content. The required harmonic is accentuated by the auxiliaryresonant circuit formed by the capacitor 10 and the portion of theinductance 3 across which it is connected. The auxiliary circuit istuned to accentuate the second harmonic but is kept slightly olf tunesince a cosine function is needed. The auxiliary capacitor 2 is chargedby diode 4 to the peak value of the alternating potential across thecircuit. It can be seen that the frequency of light modulation, that is,the rate of pulsing of the cell 1, is in this case equal to that of theapplied alternating potential. The cell potential varies in aunidirectional manner so that power factor losses are minimised. Thereduction in power factor losses is particularly useful at frequenciesabove 2 mc./s. It can also be seen that it is necessary to produce onlyabout half the voltage required to open the cell completely. The 2/)ratio is approximately l/4.5 where e is the exposure time and f is theframe interval.

The alternating potential need be applied to the cell 1 only for theduration of the photographic recording, which may be of the order of 20microseconds. Thus the valve 6 need be pulsed for this period only,enabling a high power output to be obtained with a very small powerpack. i

The sinusoidal oscillations of FIG. 3 require an amplitude equal to thatrequired to open the cell completely. The frequency of the lightmodulation is, however, twice that of the applied potential. The e/fratio in this case is approximately 1/2. 1

The oscillations shown in FIG. 4 result from the negativesuperimposition of the third harmonic upon the fundamental. Thefrequency of the light modulation is twice that of the applied potentialand the peak voltage must be equal to that required to open the cellcompletely. the e/ ratio is approximately l/4. The auxiliary circuitaccentuating the third harmonic must, of course, be exactly in tune withthe main resonant circuit.

The oscillations shown in FIG. 5 are unidirectional, thus reducing powerfactor losses, and the peak voltage of the applied alternating potentialneed be only about 20 half that required to open the cell completely.The frequency of light modulation is equal to that of the appliedfrequency and the e/f ratio is approximately 1/ 3.

A camera using an electro-optical shutter operating as illustrated inFIG. 5, the cell capacity being 60 pt. and the peak applied voltagebeing 15 kv., and frequency mc./s., can make a series of exposures ofabout millimicroseconds.

I claim:

*1. An electro-optical shutter for a high-speed camera which comprises aKerr cell whose electrodes are connected across an inductance to form aparallel resonant circuit, a driver electronic valve having its anodeconnected to a tapping along the said inductance and its cathodeconnected to earth, and a high potential lead connected to one electrodeof the Kerr cell, a capacitor being connected in series with said Kerrcell and a diode being connected in parallel across said Kerr cell.

2. A high speed camera shutter as claimed in claim 1 wherein a capacitoris connected in parallel across part of the inductance.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Jones, High Speed Photography, Chapman and Hall Ltd, London,1952 ed., pages 14 and 15 relied upon.

